Vapor-heated chamber

ABSTRACT

Heating apparatus comprises a chamber (10) in which articles to be heated (e.g. sterilized) are located via a door (16) and which can be filled with a blanket of dense hot organic vapor during a heating phase. The liquid which creates the vapor blanket is located in an annular reservoir (20) and is heated to above vaporization temperature by an electrical band heater (22).

TECHNICAL FIELD

This invention relates to a heating appliance which provides avapour-heated chamber. It has particular, but not exclusive, referenceto an ambient pressure steriliser capable of rapidly sterilising a smallcharge (e.g. up to 2 kg) of artifacts (e.g. dentist's or surgeon'stools), but can also be used for general heating applications (e.g.curing of plastics articles or vapour-phase soldering).

It is known to sterilise small artifacts in a chamber filled with thevapour of a liquid having a boiling point above 100° C. (e.g. above 150°C.), brought to boiling point in an electrically-heated reservoirforming a lower region of the chamber during a heating phase and,following a sufficient dwell time in the vapour to achievesterilisation, to cause the vapour to condense down back into thereservoir during a condensation or cooling phase, leaving the chamberfree of vapour. One example of an appliance which operates in this wayis described in GB-A-2160772.

For the fastest recycling up to sterilising temperature and back to asafe handling temperature, both the heating and condensation phases needto be as short as possible. It is known to cool the chamber externallywith air during the condensation phase to reduce the duration of thisphase, but it is important to control the cooling process so that liquidcondensate is not left on the artifacts or the trays or other platformsused to support them. It is known to locate an immersion heater directlyin the liquid to reduce the duration of the heating phase, despite therisk this brings of hot spots occurring on the heater that may take theheating liquid above a recommended safe operating temperature.

It is known (see for example GB-A-353832) to use a reservoir for theheating liquid in a steriliser which has spaced-apart inner and outerwalls defining therebetween a volume for the heating liquid.

SUMMARY OF THE INVENTION

According to this invention a heating appliance (and in particular asteriliser) including a vapour-heated chamber, a reservoir adapted tocontain a vaporisable heating liquid and heating means for the liquid tovaporise the latter and to supply the vapour to the chamber, thereservoir having spaced-apart inner and outer walls definingtherebetween a volume for the heating liquid, is characterised in thatone of the inner and outer walls is used to heat the liquid during theheating phase and the other of the walls is used for cooling the liquidduring the condensation phase.

Conveniently both walls are cylindrical walls (e.g. of circularcylindrical form) and are concentrically mounted with their common axisextending vertically. Suitably the inner wall is formed as an extensionof a domed lower wall of the chamber, the domed lower wall being convextowards the chamber. The inner wall has a smaller surface area than theouter wall and it may be thermally more efficient to use the inner wallfor transfer of heat into the reservoir and the outer wall for transferof heat out of the reservoir. Practically however, wrapping a bandheater around the outer wall is a very easy way of heating the liquidleaving the inner wall free for cooling. Air cooling is preferred, butliquid cooling (e.g. via water flows) is not excluded. Where air coolingis employed it may be desirable to improve the heat exchange coefficientof the appropriate wall of the reservoir, e.g. by the use of fins, asurface coating or a surface treatment on the reservoir wall/walls.

Where air cooling is employed, a simple fan can be located outside thechamber adjacent to the chamber walls, the air flows from the fan beingused partly to cool the chamber walls and partly to cool one or bothwalls of the reservoir. The proportion of air used for these twopurposes can remain unchanged during the condensation phase or varied(e.g. on a preset pattern) depending on the stage of cooling reached inthe condensation phase.

To further reduce the time needed for the condensation phase, means canbe provided to move vapour within the chamber so that enhancedvapour/cooled wall contact can be achieved. A rotatable vapour stirrer(e.g. a fan blade) can be located in the chamber and powered from amotor located outside the chamber.

A wide range of heating liquids can be used but certain halogenatedorganic compounds are particularly suitable heating liquids, preferablythose having a boiling point above 175° C. and vapour densities at least10 times that of air. Perfluoroperhydrofluorene (C₁₃ F₂₂) isparticularly preferred.

The size of the reservoir will be dependent on the heating liquid usedbut with perfluoroperhydrofluorene a volume of some 400 to 500 mlappears to be satisfactory. The surface area of the outer wall of thereservoir, for a charge of the volume stated above, could convenientlybe of the order of 380 cm² and the surface area of the inner wall of thereservoir of the order of 300 cm². A band heater of some 2 KW ratingwrapped around the outer wall provides a safe but fast heating with areservoir of the size just described and with forced air cooling of theinner wall to accelerate the condensation phase, cycle times of under 20minutes are possible for a 2 kg charge of artifacts sterilised to 190°C.

Stainless steel is a suitable material from which the reservoir wallscan be formed and a wall thickness of less than 1 mm (e.g. around 0.5mm) is preferred.

BRIEF DESCRIPTION OF DRAWINGS

One embodiment of heating apparatus according to the invention will nowbe described by way of example, with reference to the accompanyingdrawings, in which

FIG. 1 is a schematic view of the apparatus with the access door shownonly in part and without the decorative outer casing,

FIG. 2 is a schematic enlarged sectional view of the heating/coolingreservoir of the apparatus of FIG. 1, and

FIG. 3 is a schematic sectional view showing the cooling fan used in theapparatus of FIG. 1.

DESCRIPTION OF PREFERRED EMBODIMENT

The apparatus shown in FIG. 1 comprises a generally rightparallelepipedic chamber 10 having a rear wall 11, side walls 12, 13, atop wall 14, a base wall 15 and a front wall which is mainly constitutedby an access door 16 (shown only in part). The artifacts to be heated(e.g. dentist's tools to be sterilised) can be placed on trays (notshown) located in the chamber to slide in and out on runners mounted onthe side walls 12, 13.

A breathing tube 17 communicates with the uppermost part of the chamber10 through the top wall 14 and contains vapour condensing surfaces (e.g.glass beads) so that as air is expelled through the tube during aheating phase, any vapour or droplets of heating liquid entrainedtherewith will be collected to drain back into the chamber 10. The tube17 can be of copper to improve its performance as a condenser. A falseceiling (not shown) can be located near to the top of the chamber 10 toreduce the risk of vapour entering the breathing tube 17.

The base wall 15 is mainly constituted by a dome 18 forming the upperextremity of a cylindrical inner wall 19 of an annular reservoir 20 forheating liquid. The outer cylindrical wall 21 of the reservoir 20supports an electrical band heater 22 with a temperature-sensitive fuse23 in series with the heating element thereof.

Three temperature sensors 24, 25 and 26 are provided in the chamber andconveniently these are PTC (positive temperature coefficient)thermistors. Sensor 24 acts as a liquid level sensor in the reservoir 20and is mounted in an aperture in the inner wall 19. Sensor 25 ensuresthe access door 16 cannot be opened after the heating phase until thetemperature near the bottom of the chamber 10 has fallen to a safe level(e.g. 50°-60° C.). Sensor 26 is disposed a short distance below thefalse ceiling and is used to determine the end of the heating phasesince it senses when it is in contact with vapour at the required hightemperature. Circuitry can be employed to control the durations of theheating and condensation phases on the basis of the temperaturesrecorded by the sensors, but this is not described since it can be ofconventional design and is not relevant to the novel aspects of thisinvention.

FIG. 3 shows a fan motor 30 coupled to an external centrifugal air mover31. When the heating phase is completed, the motor 30 is energised andmoves air in a first flow path that passes over the outer surfaces ofthe band heater and the side walls of the chamber 10 and in a secondflow path that contacts the underside of the dome 18 and flowsdownwardly past the radially inner surface of the inner wall 19 andexits from the underside of the unit. A false floor can be provided toisolate the two flow paths and ensure that the air flows leaving theunderside of the dome 18 do not mix with the air flows passing over theside walls of the chamber.

In operation of the appliance shown, following loading of the chamberwith the articles to be heated and a closing of the door 16, the heater22 is energised to rapidly raise the temperature of some 450 to 500 mlof C₁₃ F₂₂ (e.g. a fluorocarbon liquid known under the trade name"Flutec PP10" of ISC Chemicals Limited) in the reservoir 20 to itsboiling point of 190°-192° C. The vapour generated, rises as a denseblanket (the vapour density of "Flutec PP10" is almost 20 (relative toair) at its boiling point and without pressurisation) in the chamber 10engulfing the articles supported therein. This causes the temperature ofthe articles to rise rapidly to the temperature of the vapour. Theheating phase can be concluded a pre-set period after the blanketreaches the top sensor 26, and the fan motor 30 can then be energised.The blanket begins to subside with vapour condensing on the rear andside walls of the chamber and on the dome 18. By controlling the timeduring which articles in the chamber are submerged in the blanket ofvapour, proper heating (e.g. sterilisation) of the articles can beensured and by controlling the rate of heat loss from the walls 11, 12,13, 15, 18 and 19, rapid cooling can be achieved (e.g. a full cycle inunder 20 minutes) without leaving residues of liquid on the articles.

Since the condensation phase requires heat to be lost from the chamber,its duration can be reduced by forcing vapour to move within the chamberoutwardly to the cooled walls. A rotatable vapour stirring device isshown at 29 in FIG. 1 and this can be powered by a motor (not shown)located outside the chamber and energised during the condensation phase.A convenient arrangement is to use a magnetic coupling between thestirring device 29 and an external driving motor (which could be themotor 30).

Almost complete return of heating liquid to the reservoir 20 can thus beachieved during the condensation phase. The heater 22 should be designednot to exceed the recommended maximum watts density for the sterilizingfluorocarbon and a 2 KW band heater having a heating area of some 350cm² appears to achieve this well in the case of "Flutec PP10". The factthat no part of the heater comes into contact with the heating liquid isa further advantage, since even in a rare fault condition, the safeoperating temperature of the liquid will not be exceeded if the fuse 23is appropriately dimensioned.

In the particular design described, provision can be made to keep thevapour blanket at the top of the chamber 10 for a predetermined periodusing a micro-processor to satisfy any regulations which might beimposed (e.g. by Health Authorities, etc. regarding sterilisation time).The micro-processor can be used to cycle the band heater on and off tomaintain the top of the blanket about a nominal position using closedloop control via the top sensor 26.

Although the invention has particular utility with reference tosterilising apparatus it will be appreciated that the use of a reservoirin accordance with this invention has many advantages, in other heatingapplications where a controlled temperature and optionally also anon-oxidising atmosphere is required. An oven similar to the chamberillustrated can be used for a wide range of heating purposes. Thus anoven for curing plastics mouldings (e.g. at temperatures upwards of 130°C.) or for effecting vapour-phase soldering (e.g. at temperaturestypically 180° C. to 220° C.) can usefully be constructed based on afluorocarbon liquid in the reservoir and other heating liquids can beused for this and other heating applications.

Among the advantages of the invention may be mentioned:

1. It is easy to design the reservoir 20 to accommodate an off-the-shelfband heater.

2. The volume of the reservoir 20 can be reduced to accommodate close tothe minimum volume of liquid needed for generating the sterilisingblanket.

3. The small surface area of liquid in the reservoir exposed when thedoor 16 is opened, reduces loss by evaporation.

4. A large surface area for heating the liquid can be obtained which isseparate from a large cooling area.

5. Shorter times for the heating phase are possible.

6. Shorter times for the condensation phase are possible.

7. Very even temperature can be maintained throughout the chamber.

8. Very fast heat transfer to/from articles within the chamber can beobtained.

9. A non-oxidising atmosphere can be available during the heating phaseby appropriate choice of heating liquid.

10. Plastics materials cured within a chamber heated in the mannerdescribed tend to have a better homogeneous structure.

11. Fast and efficient soldering in the hot vapour is easily achieved.

We claim:
 1. In a heating appliance for cyclicly heating an article to apredetermined temperature during a heating phase of a cycle and thencooling the article during a condensation phase of the cycle, whichappliance comprises a chamber having a lower part, a reservoir in thelower part of the chamber constructed to contain a vaporisable heatingliquid, and heating means for heating said reservoir to vaporise anyliquid therein to form vapour and to supply the vapour to the chamber,the reservoir having spaced-apart inner and outer walls definingtherebetween a volume for the vaporisable heating liquid,the improvementwherein said heating means comprises means to heat one of the inner andouter walls whereby to heat the liquid, said heating means beingactivated during the heating phase and said apparatus comprises coolingmeans for cooling the other of said inner and outer walls whereby tocool the liquid, said cooling means being activated during thecondensation phase.
 2. A heating appliance as claimed in claim 1,wherein both said inner and outer walls are of circular cylindrical formand are coaxially mounted with their common axis extending vertically.3. A heating appliance as claimed in claim 1, wherein said inner wall isformed as an extension of a domed floor of the chamber, the domed floorbeing convex towards the chamber.
 4. A heating appliance as claimed inclaim 1, wherein said heating means is an electrical band heater wrappedaround the outer wall of the reservoir.
 5. A heating appliance asclaimed in claim 1, comprising fan means to move the vapour within thechamber, said fan means being activated at least during the condensationphase.
 6. A heating appliance as claimed in claim 5, said fan meanscomprising a rotatable vapour stirrer within an upper part of thechamber.
 7. A heating appliance as claimed in claim 1, said coolingmeans comprising a fan outside the chamber with duct means to cause aflow of air to cool said other of the walls of the reservoir.
 8. Aheating appliance as claimed in claim 1, said cooling means comprising afan located outside the chamber, the air flows from the fan being usedpartly to cool the chamber walls above the lower part containing thereservoir, and partly to cool at least said other of said inner andouter walls of the reservoir.
 9. A heating appliance as claimed in claim1 for use as a sterilizer, said reservoir containing a volume of ahalogenated organic compound comprising the vaporisable heating liquidhaving a boiling point above 150° C., said means to heat one of thewalls comprising electric heating means for heating said volume ofliquid through said one wall of the reservoir.
 10. A heating applianceas claimed in claim 9, wherein the organic compound has a boiling pointabove 175° C. and a vapour density of at least ten times that of air.